A user's physiological signals (e.g., pulse rate or arterial oxygen saturation) can be determined by photoplethysmogram (PPG) systems. In a basic form, PPG systems can employ one or more light sources that illuminate a user's tissue and one or more light detectors to receive light that enters and probes a subsurface volume of tissue. The light sources and light detectors can be in contact with the tissue or can be remote (i.e., not in contact) to the tissue surface. The received light can include light with an amplitude that can be modulated in time as a result of interaction with pulsatile blood flow and parasitic, non-signal light that can indirectly sample pulsatile tissue volumes with an amplitude that can be modulated (i.e., “noise” or “artifacts”) and/or unmodulated (i.e., DC).
Although PPG systems measure the pulsatile blood flow to determine a user's physiological signals, these measurements may be corrupted by noise introduced by, for example, the user's motion, motions from within the user's body (e.g., tendon motion and/or muscle motions that can affect venous blood volume information), tilt and/or pull of the device, ambient light variations, or any combination thereof. While some PPG systems can utilize accelerometer measurements to correct for such noise, accelerometer measurements can be limited to the gross, periodic motion. Given that a user's motion may not be limited to gross, periodic motion, a PPG system capable of differentiating pulsatile blood volume changes from anatomical motion can be desired. In some examples, anatomical motion can be measured by measuring non-pulsatile blood volume changes.